Hard brake vibration, dealer said its ok.

[illusionaryline]

hey guys,

yesterday i got all four tyres replace, along side a wheel balance and alignment, previous to this i had been having problems with steering wheel shaking when over 90km/h, the new tyres and alignment have solved this, however 3 weeks ago when the car was at the dealership i complained about the brakes vibrating heavily when braking over 80km/h, they said the brakes are fine, i disagree. it feels really unsafe as the whole car stutters and vibrates when braking. any help or suggestion would b awesome thanks

[jnenad16]

have the rotors checked for warpage.

nenad

[rman]

Originally posted by jnenad16
have the rotors checked for warpage.

nenad

and either have them resurfaced, if there is adequate thickness, or replace them.

[Abuimad]

i had the same problem its was my disks, due to washing my car when they are at high temperatures they warped...bend really bad and i had to have the replaces

[illusionaryline]

thanks very much guys, ill head to the dealer next week.

[Renn 208]

So...I've heard both sides, that rotor warp is a myth, and that rotors can warp. Anyone have a definitive answer?

[jnenad16]

Originally posted by Renn 208
Can anyone show me what a warped rotor looks like? I haven't been able to find pics, or any measurements of warpage.
most of time, its not visible, but once you connect the gauge to it and start to spin it, you see the noticable surface variations.

nenad

[Renn 208]

Originally posted by jnenad16
most of time, its not visible, but once you connect the gauge to it and start to spin it, you see the noticable surface variations.

nenad

but is that warp, or pad deposits? I think that in particular, it's the notion that you could go through a puddle or wash your car and cause the rotors to warp that's under question.

[jnenad16]

the most common cause of rotor warpage is excessive heat generated under severe braking conditions and inadequate brake cooling. most stock setups are designed to prevent such thing. however, aftermarket brakes contribute to this because people usually upgrade only the pads with the higher friction ones, but dont often realize that the stock rotors werent designed for the extra heat that will be generated out of those pads.

the pad deposits will never leave an uneven disk surface because they will always be embedded into the rotor, and from frequent braking, they will actually become a part of the rotor's surface.

nenad

[rman]

they do warp from heat. it's called runout. they get wavy, like a potato chip!

that's not what causes the vibration - - since they have warped, the pads hit it at inconsistent pressures, almost like they are bouncing off the rotor (just for a visual). That give the rotor variance (in thickness) and that is what causes vibration.

i was once a tech at SEARS and also a service writer. not only do i know of the problems, but i can explain them as well ;)

variance of even .005" can start to have [very slight] vibration under [hard] braking

[Renn 208]

so...I asked my buddy google to help out, and found this:

from stoptech:
http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

then this from an OE engineer in response to the link above:


I respect Carroll Smith as a race car engineer, designer, and crew chief, especially since I've got 2 66 GT-350's. There is a lot of truth in a number of things that is said in his article, and some myth's as well. There is a lot that goes on within the friction material industry that is really not discussed outside of the companies, or published in SAE articles. The science is kept as a "black art" for competitive reasons. You have to sign some type of castration policy after you start. It was 25 years ago - I don't remember the details exactly.

As mentioned in the sales pitch article, I've never encountered "Warped" rotors in the way the consumer, or mechanics, perceive the term. You can stress a rotor to distort, install it with a high lateral runout due to machining tolerance stacks or a poorly machined rotor, have issues due to uneven disc castings (2 discs separated by cooling vanes assembled on a hub section is called the rotor), and a few other little funky things, but they don't warp from high temperature use, keeping the brakes on at a stop, or splashing from rain puddles.

All friction materials develop transfer to the rotor or drum. It has to happen to achieve full braking effectiveness. Cast iron (as mentioned, all rotors are made from this) by itself has a low coefficient of friction. By having a transfer layer, you develop adherent friction and a higher degree of stopping power (mue). Organic friction materials in the early 90's had a very bad case of friction transfer issues (example 90 Towncar) with caused pulsation due to the thick transfer layer of material. We were one of the many friction material suppliers tried on this platform that had the issue. It never should have had organic pads, but that was what the customer (Ford) wanted. We can be as stupid as the next supplier and make the sales until the next guy is chosen to fail.

It was actually kind of interesting. You had the driver do a sequence of specific stops until the transfer happened, then without another application, remove the rotor to see as well as measure the transfer. If you made one more brake application, the heavy transfer was gone. It was due to the rotors cooling faster then the pads, so an application with cool rotors / hot pads would generate the heavy transfer layer.

The organic compounds are only now used on light duty applications, and they are much different then they were. The majority of disc brake materials, and all on light and medium disc brake trucks, are semi-metallics. As a rule, they are very low in organic ingredients, such as cashew friction particles and the phenolic resins that bind the matrix together.

All friction materials imprint to the opposing surfaces. Always will. And this imprint wears away in short order. Friction materials are abrasive. Measure the amount of rotor wear by the time you start to feel pulsation. If this was the cause of the issue, you would feel the pulsation in short distances. You don't. You also don't keep stopping at the same exact place on the rotors. So if you ended up encountering the same conditions for a heavy imprint on the rotor, the statistics of it happening only at one point are far remote. It would occur around the entire surface. Also, many times the imprinting that one sees is actually iron oxide pitting from moisture being trapped between the pad and rotor for a number of hours when the vehicle is stopped, so don't freak out when you see this.

When cast, rotors are left in the sand form to cool down to alleviate thermal stresses. At least the OE and better aftermarket rotors are. I would never, never buy an off shore rotor. One thing that can lead to thermal stressing and the forming of carbide (hard spots) is uneven rubbing discs. You rarely find this is a Ford supplied rotor, but it has been an issue with other manufacturers (even with the same OE rotor supplier). Neon's were really bad. The thin section of the disc heats up faster then the rest of the rotor, but cools quickly. It's the rapid heating and cooling that forms the carbides.

So what type of driving conditions does pulsation or DTV normally happen in? It can happen for a number of reasons. But the majority of the time it is with vehicles that do a lot of highway driving, which is actually the coolest driving style you can have. Of course if your at GVW or towing through the mountains, you can be very hot, but vehicles that spend most of the time in city traffic usually have the hottest brakes. One or two highway stops are not going to do it.

For example of how hot our brakes get, here is what an F-250 fully loaded to GVW using the OE brake pads looks like when it runs the 2nd fade sequence during the NHTSA FMVSS 105 stopping distance certification test. First this vehicle is at GVW 8,800 lbs. The sequence of stops during this section is to run 3 baseline stops to establish the stopping ability of the braking system under average, moderate brake applications (200F Initial Brake Temperature; 40 mph; 10 fpsps (.3g) deceleration). As a reference, most people stop at 5-7 fpsps. Next, the fade sequence is performed with 15 stops from 60 mph to 0 mph; 15 fpsps; 30 sec intervals, with the first stop starting below 200F IBT. Basically as soon as you come to a full stop, you go wide open back up to 60 mph, then do the next stop. We buy a few transmissions. After the 15 stops, you immediately perform the recovery stops back at 40 mph and 10 fpsps, and compare them to the baseline stops to see if there is any delayed fade or other nasty characteristics. During the fade sequence your allowed to go up to 150 lbs pedal effort, but can't be below 5 fpsps deceleration during the last 5 fade stops.

2nd Fade sequence (You can look at it best at full screen, or better yet download it and print out the jpeg).

So why is this fool showing me this data. By reading the article, you think you get these brakes really hot. They do (the rotors are about 200F hotter then the pads), but think about the sequence that this vehicle just went through. Seven miles of 0 to 60 to 0 almost as fast as you can with a fully loaded vehicle. Now if this was a disc drum vehicle, like the pre 99's, you would be up in the 1,400F range - Rear drum brakes don't work hard. But coming off an off-ramp with brakes in the 100F normal cruising temp will only get you up into the 300-400F side of things.

What does get the vehicle into trouble is cold or off-brake rotor wear. When you going down the highway and one brake pad touches a section of a rotor due to some lateral rotor runout, that section wears down. Eventually there is enough wear that a difference of about 0.0005" exists between the inner and outer rubbing surfaces (where the inner and outer pads touch). This out of parallelism by itself can cause the pulsation that drivers feel.

Different vehicles have different sensitivities, even within a platform. The early 99 vehicles are more sensitive then the 01 vehicles. There are no magic parts that can be changed, it has to due with a number of items that are matured during production. A sensitive driver with a 99 may notice the issue with 0.0003" DVT, but the average driver / average vehicle will have the issue at about 0.0006" DTV.

Sounds like the best way to stop this is with rotors that run straight and true. Ford has tightened the machined specs of the hubs, rotors and wheels bearings over the years. It is rare for me to see a new production line rotor measuring over 0.0010" installed TIR. The service install spec is 0.0015". This is really one tight spec. It was not that long ago that rotors with 0.0050" TIR were considered OK. But even when you look at a spec of 0.0010", and realize that 0.0005" DTV wear can be felt, there is still a potential problem.

But wait a minute. Calipers pull the pistons back. It's called seal rollback (the piston square seal in the caliper's tangential groove rolls the piston back into the caliper on release) and it's usually about 0.0200". There's plenty of room between the pads and the rotor - 0.0100" on each side. In an ideal world.

First, the seal looses some elasticity over time and temperature. And there is some dirt and oxidation that can get under the boots. If you push in a new caliper's pistons with your thumbs, then try the same with an old used caliper, you'll see a big difference. Over time, that rollback may be down to 0.0100" or less. And a caliper or O-ring that was not made correctly can be worse.

But wait, 0.0100" is still plenty of room. If the sliding pins are working well the rotor will "knock back" the outer pad for clearance. If the pins hang up, the inner pad is getting all of the clearance, and the outer pad can touch the rotor.

The other factor here is the normal distortion of the wheel bearings and hub while the vehicle is turning and hitting pot holes. The rotor at the radius of the brake pads can move laterally by 0.0200" when new and to spec under these conditions. It will even move that much if the wheel / tire assembly is out of balance (there's a thought - gee, wouldn't the out of balance always be at the same plane of reference with the rotor, so the rotor keeps hitting the inner and outer pads at the same point going straight down the road). Of course there is also the acceleration force of moving that heavy caliper to one side when the outer pad and disc meet, throwing the caliper to the point that the inner pad hits the rotor, then it gets thrown back the other way. What would that do to rotor wear?

Also, these calipers use tension clips against the pad ends in the torque brackets to reduce noise. When dry and dirty, the pads may not slide back very well. In comes the concept of V-Springs about 2001. Still not as good as being cleaned and coated with silicon caliper grease.

Now that we have worn a thick and thin area of the rotor, constant stops from high speed will cause those thick areas to heat and cool rapidly, forming the carbides mentioned, along with the heat growth issue in that region. This is when you feel the pulsation getting stronger during an off-ramp brake application.

Your not going to machine those hard spots out and the pulsation can come back in a short time, sometimes right after you've left the shop. This is why some dealers do not turn rotors at all. A good mechanic who listens to the rotor being cut can tell if it has hard spots and will toss it if it does. Most will be taking the rotor off the other side of the vehicle while the rotor is being turned and miss it. Often in a machined or sanded rotor, you can see the hard stops as shiny areas.

I probably missed about a dozen things.

When the situation of organic material transfer came up, a number of companies developed ways of measuring the transfer layer. In some cases, doing the surface to surface thickness measurement, then dissolving the transfer layer or iron rotor away and measuring what remained. There still are aftermarket friction materials that have the issue, but your paying $20 for a set of those pads.

And abrasive is not always the best answer. Ask mechanics who worked on the 94-95 F-150's with the Performance Friction pads. Pads or rotors are not always the solution. And consider the out of balance tire situation would still cause more havoc with an abrasive pad.